1. Field of the Invention
The present invention relates to a method for forming a computer generated hologram to be used for optical information processing by displaying the computer generated hologram with a liquid crystal spatial light modulator.
2. Description of Related Art
Holography is a technique of three-dimensional optical image formation for recording, and later reconstructing, the amplitude and phase distributions of a coherent wave disturbance. A hologram is a photographic recording obtained by recording the interference fringes between the waves reflected from an object and the mutually coherent waves called the reference light from the same laser.
A computer generated hologram is optical information in the form of the digital data of the above-mentioned amplitude and phase distributions of a coherent wave distributions at a position for recording, and it is obtained by computer simulation on the basis of wave optics. Such a computer generated hologram is used to display an optical image with use of a liquid crystal spatial light modulator. In other words, an electric voltage applied to each pixel of the liquid crystal spatial light modulator is controlled according to the data of computer generated hologram so as to modulate spatially the transmittance or the reflectance of pixels.
In a layer of twisted nematic type liquid crystal of a spatial light modulator, the longer molecular axes of liquid crystal molecules are twisted by 90.degree. from the incident side to the outgoing side, and the polarization of the linearly polarized, incident light is rotated along the longer molecular axes of liquid crystal molecules. By applying an electric voltage to the liquid crystal molecule layer, the twist of the liquid crystal molecules decreases, and the twist of the polarizing direction decreases so that the transmittance of the liquid crystal molecule layer vary with the applied electric voltage. Thus, the transmittance is modulated spatially by controlling the applied electric voltage.
However, when the amplitude component of the incident light is modulated by controlling the applied electric voltage, the length of optical path of the transmitting or reflecting light varies with the transmittance or the twist of liquid crystal molecules according to the applied electric voltage. Therefore, the phase distortion is caused by the optical path difference between pixels divided by the wavelength of the incident light, and the phase distortion varies with the applied electric voltage. If such a phase distortion arises in optical information processing in a coherent optical system wherein both amplitude and phase of light are processed, the modulation of the amplitude component of light accompanies inevitably an undesirable change in the phase component. Thus, required optical information processing cannot be carried out by using a liquid crystal spatial light modulator.